Well, in their defence, it is the time of the year for fireworks . Still, not a good day for the ISRO...

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

ISRO seems to have made modifications to the GSLV for this flight, lengthening the rocket to carry extra fuel, and enlarging the faring to accommodate greater payload weight. The GSAT-5P communications satellite was the heaviest payload that ISRO has ever tried to launch.

Either the modifications to the rocket resulted in new vibration modes that ISRO failed to account for, or else ground crew were simply careless during pre-launch checks, failing to ensure that all control cables were adequately secured.

"The take-off was smooth and the flight was normal till 47 seconds. But trouble arose in the next three seconds, when 10 connectors located between the second and third stage (cryogenic stage) got separated, leading to the vehicle losing controllability," the sources said.

So that sounds like something could have happened with the cryogenic upper stage (CUS), that could have caused a major problem to the connectors below it. Could a cryo-propellant tank have ruptured/leaked due to aerodynamic stresses, and then disabled those connectors/cables?